Allowing and enabling D2D communications within a wireless cellular communication system associated with a frequency spectrum increases the degree of utilization of the frequency spectrum. The reason for this is that a pair of UEs communicating in D2D mode may reuse the frequency spectrum resources, such as the physical resource blocks, PRBs, of a 3GPP Long Term Evolution, LTE, system. When D2D communications use the downlink, DL, resources of a cellular communication system, existing radio resource management techniques can protect the PRBs used for DL cellular transmissions of user data or DL control information transmitted on DL control channels. In this way, user specific reference signals, like the demodulation reference signals, DMRSs, used for demodulation, which are transmitted within the same PRBs as the user data are automatically protected as well. Such techniques are possible since in cellular integrated D2D communications, the base station, e.g. the eNB, schedules and/or grants access to resources used for both D2D and for cellular transmissions within the system. In general, existing techniques that manage and coordinate resource allocation for the D2D and cellular layers focus on the user data resources and typically assume that only a small subset of the available PRBs are used by a specific D2D pair at a time.
Existing methods allow the base station or cellular access point to select the set of downlink or uplink cellular resources that should be used for D2D communication by a specific D2D pair, i.e. two devices engaged in D2D communication with each other. Dynamically determining the set of UL or DL resources that should be used for D2D communication is useful, because it can make underutilized cellular resources available for D2D communications, while protecting highly utilized cellular resources. For example, in certain situations DL resources can be highly loaded, while in other deployment scenarios, the UL resources can be the bottleneck of a cellular system.
However, even though user data sent between a base station and a user equipment may be protected in the presence of D2D communication, D2D communication within a cellular communication system may still introduce problems related to interference. The problem which has been realized by the inventors, and which will be discussed herein is the problem of D2D communication creating interference to reference signals transmitted from a base station for use by UEs. Such reference signals may herein be referred to e.g. as “cellular reference signals” or “DL reference signals”.
Different strategies are employed in wireless networks for the arrangements of reference signals in the time-, frequency- code- and antenna port domains. For example, in orthogonal frequency division multiplexing, OFDM, systems, reference signals in terms of utilizing specific subcarriers or time instances may be arranged in the time and frequency domains according to the so called block type, comb type or some other patterns, such as that shown in FIG. 1.
The exact pattern of the RSs in time and frequency can be optimized for different objectives and it also affects how the receiver of the RS, e.g. a cellular UE, can use time and frequency domain, or a combination of these, interpolations to estimate the actual channel state information, CSI for demodulation or other purposes. For example, a UE may weigh in RS measurements from the past or in other frequencies than at which the actual CSI is needed at a given point in time.
Existing technology provides no means to protect DL reference signals from interference from D2D communication when the D2D communication uses DL resources. Further, according to existing technology, UEs measuring or otherwise using the cellular reference signals do not take into account that some of the reference signals are subjected to interference from, or contaminated by, intra cell D2D transmissions. For example, in FIG. 2, cellular UE1 and UE2 may measure on cellular DL reference signals embedded in PRB1 and PRB2 that are interfered by D2D transmissions (D2D Tx 1 and Tx 2 respectively).
Existing techniques provide means to protect the user data of UE-1 and UE-2, by, for example, avoiding use of PRB-1 and PRB-2 for cellular traffic in general, or avoiding sending data to UE-1 and UE-2 using these resource blocks. In either case, the existing techniques do not address the problem of UE-1 and UE-2 using reference signals embedded in resource elements of PRB-1 and PRB-2.
This problem becomes particularly pronounced when the cellular system reallocates cellular resources that are made available for D2D traffic. For example, if the cellular BS decides to use UL resources for D2D traffic, but later it decides to switch to using DL resources for D2D traffic, DL measurements by cellular UEs may suddenly become heavily interfered by D2D traffic.